1. Field of the Invention
This invention relates to low density polymeric resin impregnated ceramic articles comprising a plurality of layers of polymer impregnated ceramic fibers e.g. tile wherein the back layer of fibers of the multilayered article is impregnated with a low temperature pyrolyzing organic polymer and to the method for making the ceramic articles. The resin or polymer impregnated ceramic articles are particularly useful as structural ceramic ablators in thermal protection systems (TPS) such as an ablative heat shield for high speed atmospheric entry vehicles. The ceramic articles of this invention are structurally stable, and can be easily manufactured to accommodate specific end-use requirements. Other uses for the polymeric resin impregnated ceramic articles of this invention include various fire retardant structures, reusable thermal protection systems (TPS) for the next generation Space Shuttle and light weight structural components for the automotive and space industries.
2. Description of the Related Art
In general, the art discloses both ablative and reusable TPS. One mechanism of rejecting heat at the front surface by a TPS material is by ablation. The ablation process absorbs a significant amount of convective heating by gas pyrolysis, decomposition and sublimation of materials at the front surface of the TPS. The remainder of the heat is conducted through the materials. For reusable TPS materials, the convective heating is rejected mostly due to re-radiation and conduction through the materials. In both cases, additional material thickness is required to keep the space vehicle structure below the critical design limit. It is the additional thickness of the material that imposes a large weight penalty for a spacecraft flying a long heat pulse trajectory, e.g., very high heat load. For these types of missions, the prior art reusable and ablative TPS use thick and consequently heavy refractory-fiber “tile” insulation to limit heat conduction to the structure. Both the ablative and reusable TPS are thick and therefore the heat shield weight occupies a large fraction of the total vehicle weight. This increased weight limits the weight of scientific payload. When using tile TPS, the thickness makes the tile less flexible and increases material cost. This requires either a more rigid structure, if the tile is direct-bonded, or a strain isolating pad.
More specifically, ablative polymers and polymeric composites comprising ceramic fibers serve an important function in aerospace technology. The composites protect aerodynamic surfaces, propulsion structures, and ground equipment from the degradative effects of very high temperatures. This protection is accomplished by a self regulating heat and mass transfer process known as ablation. High density ablators having a density of about 1.1–1.9 g/cc (68.6–118 lbm/ft3) have been developed with various polymers such as the epoxy, phenolic, and silicone reinforced with fibers such as asbestos fibers, graphite cloth, silica cloth, etc. by known processes. Low density ablators, for example, comprise polymers, silica or phenolic microballons filled with chopped ceramic fibers and/or a honeycomb structure for reinforcement. The principal heat protection of these ablators is provided by the polymer and high level of carbon char formed as a residue during the ablation process. The heat dissipation is due to heat absorption from depolymerization and gas pyrolysis, re-radiation from the char layer, and convective heat blockage from pyrolysis gas blowing in the boundary layer.
During the early years of Space Shuttle heat shield development, passive transpiration systems were proposed. The system included a low density, high temperature ceramic matrix, such as silica, carbon, potassium titanate, or graphite, impregnated with coolants such as polyethylene, or an epoxy, acrylic, or phenolic. The passive transpiration systems increases the heat rate capability of the ceramic substrate by addition of an organic coolant which functions as a transpirant. The disadvantage is that the high density of the final product increases the overall thermal conductivity of the system. In addition, because the organic coolant fills the void volume of the ceramic fiber matrix, the organic coolant acts as an effective conduction path. Conventional ablators are generally manufactured in a process wherein the polymers and other components, such as the microballons and the reinforced fibers, are uniformly mixed and cured. These products have a uniform density, however, which is a disadvantage in economizing the vehicle's weight. One of the primary purposes of this invention is to decrease the overall TPS weight by having a density gradient along the heat shield thickness, e.g., high density at the outer surface where needed and very low density near the vehicle structure. Another purpose of this invention is to provide ablating performance on the outer surface and a transpirative coolant on the inner surface.
Specific details regarding ceramic heat shields and ablative structures are disclosed, for example, in various U.S. patents. U.S. Pat. No. 4,713,275 relates to a ceramic tile for use in a thermal protection system, employing a ceramic cloth having additional ceramic material deposited therein. U.S. Pat. No. 4,804,571 relates to a thermal protection system for reentry vehicles or high speed aircraft including multiple refractory tiles of varying thickness defined by thermal requirements at the point of installation. U.S. Pat. No. 4,031,059 relates to low-density ablators comprising a siloxane resin and a low-density filler including ground cork, silica or glass microspheres and hollow phenolic resin microballons. The ablator may further contain carbon and/or silica fibers. U.S. Pat. No. 4,100,322 relates to a high thermal capacity fiber-resin-carbon composite ablator having a polymer resin filler. The composite is prepared by impregnating a woven fabric of carbon or graphite yarn with a resin, curing the resin, pyrolyzing the impregnated fabric and re-impregnating the resulting fiber-porous carbon char composite with resin. U.S. Pat. No. 4,605,594 relates to a ceramic article including a woven ceramic cloth having a non-porous core and a porous periphery prepared by treating with an acid. The porous periphery can be infiltrated with materials such as a metal, a metal oxide, a catalyst and an elastomer. The articles can be used as fiber optic elements, catalyst supports and heat resistant tiles for aerospace purposes. U.S. Pat. No. 5,154,787 describes a method of manufacturing prepreg mats. A prepreg strand formed of inorganic fibers impregnated with a thermoplastic binder or a ceramic matrix powder is heated, cooled and compacted to fuse into a uniform, dense prepreg mat. U.S. Pat. No. 3,533,813 relates to a low density, nonstructural ceramic employing a porous ceramic support in combination with organic fillers. The process includes burning off the organics to form pores within the ceramic. This process reduces the mass of the composite, thereby reducing its density while maintaining inherent strength. U.S. Pat. No. 5,536,562 describes low density resin impregnated ceramic articles and a method for making the same.